rived at B, the planet will appear at M; and in the fame manner when at CD and E, it will be seen among the ftars at NRT; therefore, while the earth moves over the large part of the orbit A B C DE, the planet will have an apparent motion from L to T, and this motion is from weft to east, or the fame way with the earth; and the planet is faid to move direct, or according to the order of the figns. When the earth is near to A and E, the point of contact of the tangent to the earth's orbit, the planet will be ftationary for a fhort space

of time.

When the earth moves from E to H, the planet feems to return from T to N; and while it moves from H to A, it will be retrograde to L, where it will again be ftationary: and fince the part of the orbit which the earth defcribes in pasfing from A to E, is much greater than the part EHP, though the fpace T L which the planet defcribes in direct and retrograde motion is the fame, the direct motion from L to T must be much flower than the retrograde motion from T to L.

When the earth is at C, a line drawn from C through S and P to the ecliptic, fhews that Mars is then in conjunction with the fun. But when the earth is at H, a line drawn from H through P, and continued to the ecliptic, would terminate in a point oppofite to S; therefore in this fituation Mars would be in oppofition to the fun. Thus it appears that the motion of Mars is direct when in conjunction, and retrograde when in oppofition.

The retrograde motions of the fuperior planets happen oftener, the flower their motions are; as the retrograde motions of the inferior planets happen oftener, the fwifter their angular motions; becaufe the retrograde motions of the fuperior planets depend upon the motions of the earth; but those of the inferior on their own angular motion. A fuperior

fuperior one is retrograde once in each revolution of the earth; an inferior one in every revolution of it's own.

OTHER PHENOMENA OF THE SUPERIOR PLANETS.

The fuperior planets are fometimes nearer the earth than at other times; they alfo appear larger, or fmaller, according to their different distances from us. Thus fuppofe the earth to be at C; if Mars be at P, he is the whole diameter of the earth's orbit nearer to us, than if he were at V, and confequently his difc muft appear larger at V than it would be at P. In other places, the diftances of Mars from the earth are intermediate.

The fuperior planets going round the fun in larger orbits than the earth, turn much the greater part of their enlightened hemifphere towards it, and therefore appear round like the full moon, except Mars, who fometimes appears like the moon at a little diftance from the full.

They alfo move in an ellipfe, having the fun in the center; the areas defcribed are proportional to the times.

They are fometimes nearer to, fometimes further from the earth, and their apparent diameter is found to vary according to the difference in their distance.

OF THE SECONDARY PLANETS, OR SATELLITES.

As the moon turns round the earth, enlightening our nights, by reflecting the light the receives from the fun, fo do other fatellites enlighten the planets to which they belong; and as it keeps company with the earth in it's annual revolution round the fun, fo do they feverally accompany the VOL. IV.

H

planets

L

planets to which they belong in their feveral courfes round that luminary. Jupiter has four fatellites, Saturn feven, the Georgium Sidus two.

The existence of all the fatellites except the moon would have been unknown to us without the use of the telescope.

The fatellites are diftinguished according to their places, into firft, fecond, &c. the first being that which is nearest the planet.

The fatellites revolve round their primaries in elliptic orbits, the primary planets being in the focus.

The orbits of all Jupiter's fatellites are nearly but not exactly in the fame plane, which produced makes an angle with the plane of Jupiter's orbit of about 3°. The fecond deviates a little from the reft.

The planes of the orbits of the secondary planets produced, interfect the heliocentric orbits of their primaries in two oppofite points, which are called their nodes. The planes of the orbits of the fatellites of Jupiter and Saturn produced, interfect the ecliptic in two oppofite points: thefe points of interfection, to diftinguish them from the other, may be called the geocentric nodes of the fatellites. The orbits of Jupiter and Saturn are fo fmall in comparison of the fphere of the fixed ftars, that the places of their fatellites nodes are not fenfibly altered by their primaries being in different parts

of their orbits.

The orbits of all Saturn's fatellites, except the 5th, which deviates from the reft feveral degrees, are nearly in the fame plane. They are nearly parallel to the plane of the equator. The orbit of Saturn's 5th fatellite makes an angle with the orbit of his primary of 13° 8'.

A fatellite in one of it's nodes, feen from it's primary, appears in the orbit of it's primary: in

all

all other parts of it's orbit, it has latitude feen from it's primary.

Every circle, whofe plane produced paffes through the eye, appears a ftrait line: every circle viewed obliquely will appear an ellipfis, more or lefs wide, according as the eye is more or lefs elevated above the plane of the circle. The orbit of a fatellite feen from the earth, when it's primary's heliocentric place is in his fatellite's true node, and the earth in it's geocentric node, appears a ftrait line: when the primary is in any other part of his orbit, the fatellite's orbit will appear an ellipfis, whose shortest axis increases, the further the primary is from the node of the fatellite.

The earth's orbit is fo fmall in comparison of the orbits of Jupiter and Saturn, that whatever part of her orbit the earth is in, when these planets are in their fatellites true nodes, their fatellites will appear to defcribe lines very near to ftrait ones.

When a fatellite is in it's fuperior femicircle, i. e. that which is furtheft from the earth, it's geocentric motion is direct: when in it's inferior femicircle, i. e. that nearest our carth, it's geocentric motion is retrograde. Both thefe motions feem quickest, when the fatellite is nearest the center of the primary, and flower when they are more diftant; at the greateft diftance they appear ftationary for a fhort time.

Since the distance of Jupiter and Saturn from our earth is very great, and our eye is never elevated much above the planes of their orbits, every fatellite of Jupiter or Saturn feen from our earth, will appear always near it's primary, and to have an ofcillatory motion, fometimes in a ftrait line, fometimes in an elliptic curve, going from it's primary, and returning to it again, firft on one fide, and then on the other. The fatellites of Jupiter

H 2

1

or

or Saturn will fometimes be hid from us by their primary, fometimes pafs between us and their primary, fometimes a fatellite will pafs between us and another fatellite.

The fatellites and their primaries mutually eclipfe each other, but there are three cafes in which the fatellites difappear to us.

The one is, when the fatellite is directly behind the body of it's primary, with refpect to the earth; this is called an occultation of the planet.

Another is, when it is directly behind it's primary, with refpect to the fun, and fo falls into it's fhadow, and fuffers an eclipfe, as the moon, when the earth is interpofed between that and the fun.

The laft is, when it is interpofed between the earth and it's primary; for then it cannot be diftinguifhed from the primary itself.

It is not often that a fatellite can be difcovered upon the difc of Jupiter, even by the beft telefcopes, excepting at it's firft entrance, when by reafon of it's being more directly illuminated by the rays of the fun, than the planet itself, it appears like a lucid fpot upon it; fometimes however a fatellite is feen paffing over the difc like a dark fpot; this has been attributed to fpots on the furface of the fatellite, and that the more probably as the fatellite has been known to pafs over the dife at one time as a dark spot, and at another time to be fo luminous as only to be diftinguished from the planet at it's ingrefs and egrefs. The beginnings and endings of thefe eclipfes are cafily feen by a telescope, when the planet is in a proper fituation; but when it is in conjunction with the fun, the brightnefs of that luminary renders both the planet and the fatellite invisible.

By obferving the eclipfes of Jupiter's fatellites, it was difcovered that light is not propa

gated